The button was falling off of the shirt. Quite literally clinging on by a thread!
I used some blue cotton and the existing button to sew it back onto the shirt.
I love this shirt and have never been able to find one similar. It was bought vintage.
I think simple things like a lost button are so easy to fix, and it really adds to the personality of the item. When I wear what looks like 'just a shirt' to other people, I know it's got a part of my determination for it to survive.